The present invention relates to an amplifier stage with collector output.
As is known, in amplifier stages with collector output it can be important to prevent any transistor included in the amplification chain from reaching high saturation, so that the current gain of each transistor drops to very low values.
This behavior in fact causes a decrease of the switching speed of the amplifier, when the transistors switch from a saturation condition to linear operation. If the amplifier is negatively fedback, in the regions proximate to the outputs of the transistors which switch from saturation to linear operation, undesired transients may occur.
Such operation at high saturation may also occur for example in the case of severe imbalance at the input. To this end, let's consider, for example, the schematic diagram of an amplifier with low drop-out (that is wherein the difference between the available supply voltage and the maximum peak-to-peak output voltage is low) illustrated by way of example in FIG. 1. In this figure the current sources I.sub.1 and I.sub.2 represent the input stage, generally composed by a voltage/current converter stage. The transistors Q.sub.1 -Q.sub.4 and the constant current source Q.sub.7, together with the resistor R.sub.1, represent the driving circuit, while Q.sub.5 and Q.sub.6 constitute the two final transistors connected between two reference potential lines V.sub.CC and -V.sub.CC and defining between themselves, at point D, the output of the amplifier which is connected to a load R.sub.L.
In this known circuit, an imbalance of the currents I.sub.1 and I.sub.2 causes an imbalance of the entire stage. In particular if it occurs that ##EQU1## the transistor Q.sub.2 is saturated, and similarly, with the transistor Q.sub.2 saturated, if the relation ##EQU2## is verified Q.sub.6 is saturated.
In the same manner, if the current I.sub.2 is sufficiently higher than the current I.sub.1, the transistors Q.sub.7 and Q.sub.5 are saturated.
The saturation levels of the indicated transistors, not being controlled, may be very high, that is the current gain of the saturated transistors may drop to very low values, with the previously described negative consequences.
To avoid high saturation of the transistors, it is already known to provide Schottky diodes connected between the base and the collector of each trnsistor subject to high saturation, so as to remove part of the base current and prevent an excessive saturation of the protected transistor. However, even this solution is disadvantageous since the Schottky diodes connected to the final transistors, in order to be able to conduct even high currents at low voltage, may require a large area (which is not desirable in the case of integrated systems with high integration level) and furthermore the integration of said diodes in an integrated circuit requires a greater number of masks, and therefore entails a greater complexity in production and greater costs of the entire amplifier.
Other known solutions make use of structures comprising a transistor and a diode approriately connected to the transistor to be protected and to the input stage. However, even these solutions are not satisfactory due to the high number of components and to the stability problems of the loop formed by each transistor with the respective antisaturation circuit.